Typically, the irradiance of the beam coming out from the laser resonator has a Gaussian profile. Laser-beam shaping is the process of redistributing the irradiance and phase of a beam of optical radiation. The irradiance distribution defines the beam profile, such as Gaussian, multimode, annular, rectangular, elliptical, or circular. For different application of laser material processing, different irradiance profile such as flat-top beam, Bessel beam or annular beam is needed.
Known prior art optical systems for laser beam shaping include U.S. Pat. No. 6,295,168 issued to John Allen Hoffnagle and Carl Michael Jefferson on Sep. 25, 2001 teaches a three lens refractive optical system that converts a laser beam to a collimated flat-top beam.
Muhammad Arif, Meer M. Hossain, Ahad S. Awwal and Muhammad N. Islam, Applied Optics, Two-Element Refracting System for Annular Gaussian-to-Bessel Beam Transformation, vol. 37, No. 19, Jul. 1, 1998, pp. 4206-4209 which discloses a refracting system using two lenses to convert an annular Gaussian laser beam into a circular Bessel beam.
D. Zeng, W. P. Latham and A. Kar, Optical Engineering, Shaping of annular laser intensity profiles and their thermal effects for optical trepanning, Vol. 45(1), January 2006, pp. 14301-14301-9 discloses an optical system for transforming a Gaussian laser beam into an annular beam of different intensity profiles. Such profiles include half Gaussian with maximum intensity at the inner or outer radii of the annulus, and full Gaussian with maximum intensity within the annulus.
Jinsong Liu, Martin Thomson, Andrew J. Waddle and Mohammed R. Taghizadeh, Optical Engineering, Design of diffractive optical elements for high-power laser applications, Vol. 43(11), 2004, pp. 2541-2548 teaches a modified iteractive Fourier transform algorithm for designing diffractive optical elements for far-field free-space laser beam shaping.
Yasser A. Abdelaziez, Partha P. Banerjee, and Dean R. Evans, Applied Optics, Beam shaping by use of hybrid acousto-opties with feedback, Vol. 44(17), June 2005, pp. 3473-3481 discloses beam shaping through adaptive feedback in an acoustic-optic device with electrical feedback by using experimentally determined parameters.
Using prior art techniques for laser microvias drilling in electronic packaging, during blind microvias drilling, smear residue is formed along the walls of the hole and at the bottom of the embedded copper pad in multilayer copper-polymer structures for Gaussian beam, flat-top beam or annular beam. To remove the residue, another cleaning process is needed.
What is needed to sole the problem is a pitchfork beam to produce residue free holes, reduce the production cost and simplify the procedures. Preferably, refractive optics are used in the optical system instead of diffractive optics and non-linear optics, because they are easy to manufacture and has low cost.